Our long-term objective is to understand the molecular mechanisms cells use for removing DNA damage. We have recently revealed a novel feature of DNA repair; ultraviolet light-induced pyrimidine dimers are selectively removed from the transcribed DNA strands of active gene in mammalian cells and E. coli. Efforts will now focus on characterizing the mechanism of strand- specific repair. Studies in E. coli will be featured because of the availability of genetic and biochemical tools. Approaches to be taken include the following: Repair of the lactose operon will be studied in relevant mutants to identify genes required for strand- specific repair and test models of possible mechanisms. Transcription in the operon will be measured to test the hypothesis that strand-specific repair is important for cell survival after UV-irradiation because it helps maintain levels of critical transcripts. The influence of transcription rates on repair rates the location of repair within the operon, and the removal of chemical damage will be studied to test the hypothesis that lesion-blocked RNA polymerase complex serves as a signal for strand- specific repair. Repair of ribosomal and transfer RNA genes will be studied to determine if strand-specific repair is influenced by the processing or conformation of the RNA as it is being transcribed. Models of strand-specific repair will be tested in vitro using purified nucleotide excision repair proteins. Our plan is to identify the participants and learn the rules that govern strand-specific repair in E. coli. Results obtained in this system will then be used to direct our investigation in other systems.